The present invention relates to a method for precision shaping of wafer materials or, more particularly, to a novel and improved method for shaping wafer materials such as silicon semiconductor wafers used in electronics industries with high dimensional precision and high production efficiency.
As is well known, various kinds of single crystal materials are widely used in electronics industries such as high-purity silicon and germanium as a semiconductor material and gadolinium gallium garnet (hereinafter abbreviated as GGG) as a magnetic bubble-memory device in computers. These single crystal materials are usually used in a form of very tiny chips. Such tiny chips of single crystal materials are prepared by first slicing a rod-like so-called ingot or boule of the single crystal as grown into wafers of a thickness of several tenths of a millimeter and then cutting the wafers into chips.
Along with the progress in the electronics technology, requirements for the dimensional accuracy of these materials are growing more and more in order not only to facilitate the quality control of the finished electronic devices but also to improve the production efficiency. For example, the diameter of wafer materials as is required to be controlled within .+-.0.2 mm or, preferably, within .+-.0.1 mm. In addition, it is a common practice in handling of wafer materials that circular wafers are provided with one or more of so-called orientation flats to facilitate mounting of them on a cutting machine. An orientation flat is a portion of straight periphery obtained by cutting off a crescent-shaped portion from a circular wafer.
The conventional procedure for shaping wafer materials satisfying the above mentioned requirements both in dimensional accuracy and shape is first to grind the rod-like ingot or boule of the single crystal until the rod has a cross section exactly in conformity with the shape and dimensions of the finished wafers and then to slice the thus ground rod into wafers on a slicing machine.
The above described process for wafer shaping has problems in several aspects. Firstly, as is often the case when the direction of the crystallographic axis in the wafer is of utmost importance, the plane of slicing is not always exactly perpendicular to the axis of the single crystal rod according to particular needs in the crystallographic orientation of the finished wafers. In such a biased slicing, the wafer as sliced is no longer circular but elliptical so that the difference in the lengths of the longer and shorter axes sometimes exceeds the permissible error in the diameter of the circular wafers. Assume that, for example, the single crystal rod has a diameter of 100 mm and the plane of slicing is biased by 4.degree. from the plane perpendicular to the rod axis, then the longer axis of the elliptical wafer as sliced is about 0.2 mm longer than the shorter axis so that the maximum permissible error of .+-.0.2 mm is unavoidably exceeded in principle.
Secondly, due to the brittleness of single crystal materials in general, chipping of the wafer edges takes place frequently in the step of slicing badly decreasing the productivity since such a chipped wafer is no longer acceptable as a commercial product.
Thirdly, some users of the wafer materials require that each of the wafers is chamfered at the periphery so as to avoid accidental chipping of the wafers in their handling. This step of wafer by wafer chamfering is very time-consuming.